Cancer is a broad group of multiple diseases, all involving unregulated cell growth. In cancer, cells divide and grow uncontrollably, forming malignant tumours. In 2008 approximately 12.7 million cancers were diagnosed and 7.6 million people died of cancer worldwide. Cancers as a group account for approximately 13% of all deaths each year with the most common being: lung cancer (1.4 million deaths), stomach cancer (740,000 deaths), liver cancer (700,000 deaths), colorectal cancer (610,000 deaths), and breast cancer (460,000 deaths). This makes invasive cancer the leading cause of death in the developed world and the second leading cause of death in the developing world.
There are more than 100 different types of cancer, the name for each is derived from the tissue or organ from which it originates. Determining the causes of different cancer types is a complex process as it is generally acknowledged that cancer formation is a multi-faceted process. Cancers are primarily an environmental disease with 90-95% of cases attributed to environmental factors and 5-10% due to genetics. The chances of surviving the disease vary greatly according to the type and location of the cancer, and the extent of disease at the commencement of treatment.
Metastasis, or metastatic disease, is the spread of a cancer from an originating tissue or organ to another tissue or organ. The cells which constitute the primary cancerous tumour commonly undergo metaplasia, followed by dysplasia and then anaplasia, resulting in a malignant phenotype. This malignant phenotype allows for intravasation into the circulation, followed by extravasation to a second site for tumourigenesis. After the tumour cells have migrated to another site, they re-penetrate the vessel or walls and continue to multiply, eventually forming another clinically detectable tumour (secondary tumours). Whilst treatment regimens and therapies for primary tumours are much better understood, with improved efficacy and success rates, and whilst some types of metastatic cancer can be cured with such current treatments, most metastatic cancers show poor response. Treatments for metastatic disease do exist, such as systemic therapy (chemotherapy, biological therapy, targeted therapy, hormonal therapy), local therapy (surgery, radiation therapy), or a combination of these treatments. However, most often the primary goal of these treatments is to control the growth of the cancer or to relieve symptoms caused by same. It is therefore generally considered that most people who die of cancer die of metastatic disease.
For example, breast cancer is the most common cancer in the UK and the most prevalent cancer in women worldwide (there were 1.38 million new cases diagnosed worldwide in 2008 accounting for 23% of all new cancer cases). The relative survival of breast cancer patients has increased dramatically over the last 35 years, with localised disease largely considered to be curable, however, up to 20% of patients are likely to develop metastatic disease which has poor prognosis. Breast cancer tumours show distinct and reproducible subtypes of breast carcinoma associated with different clinical outcomes. ERBB (Her2)-positive breast cancers, which constitute around one third of all breast tumours, have a particularly poor prognosis, exhibiting resistance to first line anti-cancer drugs, and frequently developing metastatic disease—the most common cause of patient death. This particular clinical subtype is therefore an aggressive form of breast cancer with increased incidence of metastasis and consequently poor prognosis.
Therefore improved understanding of cancer progression towards aggressive metastatic forms and tumour cell-specific molecular pathways is necessary to improve and lead to new therapies. However, there are various difficulties associated with targeting metastases and discovering novel molecular targets. The differences between the early stages and metastases development require novel targeted therapy that will differ from targeted therapy for primary tumours. Moreover, the target gene has to be detectable in the disseminated tumour cells or in the primary tumour before metastases. Due to the potential latency period between primary tumour development and metastatic disease, targeted therapy requires administration for prolonged periods with fewer side effects than conventional cancer therapies.
NF-κB (nuclear factor kappa-light-chain-enhancer of activated B cells) is a protein complex that controls the transcription of DNA, and is involved in cellular responses to stimuli such as stress, cytokines, free radicals, ultraviolet irradiation, oxidized LDL, and bacterial or viral antigens. Members of the NF-κB family can both induce and repress gene expression through binding to DNA sequences, and regulate numerous genes that control programmed cell death, cell adhesion, proliferation, immunity and inflammation.
A connection between inflammation and carcinogenesis has been known for a long time, and it is therefore known that NF-κB provides a link between inflammation and cancer progression. Further, NF-κB is widely used by eukaryotic cells as a regulator of genes that control cell proliferation and cell survival. As such, many different types of human tumours have deregulated NF-κB: that is, NF-κB is constitutively active. Deregulated NF-κB has been documented in many cancers, including solid cancers such as breast, melanoma, lung, colon, pancreatic, oesophageal, and also haematological malignancies. For example, it has been shown that increased NF-κB activation was evident in 86% of HER2+/ER− breast cancers and in 33% of basal like cancers, which are associated with a shortened disease-free interval, poor survival and resistance to cancer therapy (1). Moreover, NF-κB activation in tumour cells, tumour-associated stromal and endothelial cells is thought to play a role in tumour progression and invasion (2).
In tumour cells, NF-κB is active either due to mutations in genes encoding the NF-κB transcription factors themselves or in genes that control NF-κB activity (such as IκB genes); in addition, some tumour cells secrete factors that cause NF-κB to become active. Blocking NF-κB can cause tumour cells to stop proliferating, to die, or to become more sensitive to the action of anti-tumour agents. Thus, NF-κB is the subject of much active research among pharmaceutical companies as a target for anti-cancer therapy, and numerous inhibitors of NF-κB and inducers of NF-κB are available.
B-cell Lymphoma 3 (Bcl-3) is a proto-oncogene modulating NF-κB signalling, which was first identified as a chromosome translocation in B-cell chronic lymphocytic leukaemia. Deregulated Bcl-3 over-expression has been reported in numerous tumours including several leukaemias and lymphomas, such as anaplastic large cell lymphomas (ALCLs), classic Hodgkin lymphomas (cHL) and non-Hodgkin's lymphoma (3&4). Additionally, deregulated expression has also been observed in solid tumour cancers, such as breast cancer, nasopharyngeal carcinoma, and hepatocarcinomas (5-7).
A role for NF-κB and Bcl-3 in metastatic colorectal cancer has also been shown, where it was observed that NF-κB activation occurs prior to metastatic spread (8). Notably, Bcl-3 expression was also observed in normal and tumour tissue, but a correlation between nuclear Bcl-3 and patient survival was observed. Bcl-3 expression has also been found to be increased in breast cancer cell lines and patient breast cancer samples versus non-tumorigenic cell lines and normal adjacent tissue, respectively (9). Cells overexpressing Bcl-3 also resulted in a significantly higher number of tumours which supports the role for Bcl-3 in breast cancer progression (10).
The underlying oncogenic function of Bcl-3 has never been fully elucidated. However, established thinking based on experiments performed on cancer cell lines in vitro is that it has a role in increased cellular proliferation and cell survival.
In contrast, we have previously shown that Bcl-3 specifically promotes the formation of metastasis of ErbB2 breast cancer driven tumours (11). Although primary tumour growth in the Bcl-3 deficient ErbB2 (MMTV/neu) murine model was not affected, it was shown that the occurrence of developed lung metastasis from a primary breast tumour was significantly reduced by 40%. Moreover, a significant reduction in mitotic index and apoptosis was observed in secondary tumour lesions but not in primary tumours. Furthermore, through gene expression knock down studies, it was shown that deletion of Bcl-3 resulted in an 80% decrease in lung metastases, which was attributed to loss of cell migration but importantly with no effect upon normal mammary function or overall systemic viability. The implication from these observations, and supported by leading thinkers in the field, is that specific targeting of individual NF-κB subunits or their co-activators may be a more beneficial therapeutic strategy than suppressing their upstream regulators which appear to exhibit detrimental systemic toxicity. This therefore suggests Bcl-3 may represent a suitable therapeutic target for preventing cancer metastasis and secondary tumour formation.
Bcl-3 modulates transcription through binding to the proteins p50 and p52 from the NF-κB family. We have found that Bcl-3 function can be inhibited by disruption of this binding and that Bcl-3 suppression results in a decrease in NF-κB activation, cell migration and proliferative capacity. Using molecular modelling of the Bcl3 protein bound to its cognate NF-κB protein partners, we have identified a novel pharmacophore on the Bcl3 protein, which influences its interaction with the NF-κB proteins.
We have identified compounds which are capable of suppressing Bcl3-NF-κB protein interactions, inhibiting NF-κB signalling and attenuating the cellular characteristics contributing to the metastatic phenotype observed in vivo. These compounds are therefore useful for the treatment or prevention of cancer, especially metastatic disease and secondary tumour formation.